Lice belong to the order Phthiraptera, and are the only truly parasitic group amongst the exopterygote insects. As permanent ectoparasites of most birds and mammals they exhibit a remarkable level of host specificity which is unparalleled in most other metazoan parasites. To date there are more than 3000 known species of lice and yet many more remain undescribed. With the possible exception of those species that impinge on the activity of humans and their livestock, the true biology of this cryptic group of insects remains obscure. The order Phthiraptera has been traditionally divided into two groups according to their different feeding habits: the chewing lice or “Mallophaga”, and the Anoplura, colloquially known as the “sucking lice”.
Chewing lice with their large head and mandibles comprise the largest group with some 2900 species, while the Anoplura comprise some 500 species. The Anoplura are restricted to mammals and feed using maxillae positioned at the end of a snout-like protrusion to pierce the skin. Feeding solely on blood they remain at the feeding site causing localized skin irritations to their host. Because of this they are the vectors to a number of blood borne diseases. This group includes the human louse Pediculus humanus, consequently they are probably the most well studied louse group.
Because of their potential to spread disease lice have been studied extensively with respect to control. Lice infestation has traditionally been treated using pesticides or pediculicides comprising nervous system toxicants capable of inhibiting or over-potentiating synapse-synapse and/or neuro-muscular junction transmission, many acting specifically as acetylcholinesterase inhibitors.
Representative examples of pesticides include: 1) chlorinated phenyl and cyclodiene compounds such as DDT, chlordane, heptachlor, and aldrin and dieldrin; 2) the carbamate esters carbaryl, carbofuran, aldicarb, and baygon; 3) organic thiophosphate esters such as diazinon, Malathion, parathion, and dicapthon; and 4) the synthetic pyrethroids allethrin, permethrin, resmethrin, and fenvalerate.
While these and other pediculicides have been used extensively and to good effect, they present risks to human and animal health. For example, many of the regularly used pediculicides present some direct risk to human health through residual toxicity, i.e. through direct human contact with pesticide residues. Other pesticides produce volatile toxic vapours, which can cause skin irritation on absorption or ingestion. In addition, many pediculicides present indirect risks to human health in the form of environmental pollution, most notably pollution with persistent, halide-substituted organics, which accumulate in the fat stores of food fish and other animals. These problems have led to complete bans on the use of some agents eg, DDT, chlordane, heptachlor, aldrin, and dieldrin.
There is also increasing concern that many of the known pediculicides are becoming less effective than previously experienced. For example, many of the known pediculicides have been noted to be ineffective in killing the ova or nits of lice. Thus, the use of these pediculicides results in a reinfestation of the hair or skin as soon as the ova hatch, since the treatment was ineffective in controlling and killing the ova. Accordingly, the combination of health and environmental concerns coupled with the less effectiveness of known pediculicides has resulted in a need to develop an effective treatment for both adult lice and their ova.